1. Field of the Invention
The present invention relates to a melting temperature adjustable metal thermal interface material (TIM) applicable to an interface between a microelectronic packaging component and a heat dissipation device, so as to facilitate the heat dissipation of the microelectronic component.
2. Description of Related Art
Due to the trends of multifunction, high speed, high power and/or miniaturization, microelectronic components such as LEDs having high luminance, digital signal processors and central processing units (CPUs) generate a higher heat flow, presumably causing the components' junction temperatures to exceed save operation limits. In order to prevent overheating problems of chips, the newly developed microelectronic components have been designed toward multiple cores, dynamic voltage and/or frequency scaling, or miniaturization in integrated circuits. Thereby, the aforesaid overheating problems such as a local hot spot on the chip can be reduced. According to the design difference of various microelectronic components and their dynamic operating powers, the junction temperature or the heat flow of the chips (or the dies) may vary at any time. Said requirement for the heat dissipation of the different chips brings forward diversity and development of innovative technology with respect to the microelectronic dissipation materials and the heat dissipation devices. Among the aforesaid materials, a TIM applicable to an interface between the microelectronic component and the heat dissipation device conducts the heat through a surface contact with said interface, such that the heat can be transferred effectively to the heat dissipation device such as a substrate, a heat spreader, or a heat sink. Air gaps or voids frequently appear between the interface of the microelectronic component and the heat dissipation device, and thus hinder heat conduction. The TIM characterized by fluidity or a phase change i.e. being softened or melted to a liquid state when heated is capable of filling the air gaps or the voids, so as to reduce the thermal resistance between the microelectronic component and the heat dissipation device and to improve the heat dissipation.
The above-mentioned TIMs are mostly polymer compounds, such as thermal greases and phase change materials, and the TIMs are mainly composed of poor thermal conductive polymer matrices and inorganic fillers, as disclosed in U.S. Pat. Nos. 5,981,641, 5,904,796, and 6,311,769. To further improve the heat dissipation performance of the TIMs and to prevent the polymer compounds from being deteriorated on account of thermal cycling or an ultraviolet radiation, low melting point alloys (LMAs) that are highly thermal conductive as opposed to the polymer compounds have been preferably utilized in recent research of the TIMs. Said TIMs are in a solid state under room temperature and can be melted to the liquid state when heated.
However, different microelectronic components have different heat flows. With variable dynamic operating powers, the newly developed microelectronic components possess various heat flows and different junction temperatures at any time. Because of said operation characteristics, in practice, the phase change temperatures of the phase change materials are generally designed to fall in a range of 45° C.-60° C. to meet the requirement for the heat dissipation. Although the LMAs having initial melting temperatures between 43° C. and 58° C. and including elements such as Bi, In, Pb, Sn, Cd, and so on have been well developed and applied for a period of time, the LMAs are not in compliance with the “Restriction of the use of certain hazardous substance in EEE (ROHS)” regulated by the European Union, for the hazardous elements such as Pb, Cd and the like are included in said LMAs.
The idea of utilizing the LMAs as the TIM was first proposed by Cook and Token (with reference to “A Novel Concept For Reducing Thermal Resistance” in Journal of Spacecraft, Vol. 21, No. 1122-124, published in 1984) and by U.S. Pat. No. 4,384,610 (entitled “Simple Thermal Joint”, issued in 1983). In subsequent patents of applying the LMAs to the TIMs in complicance with ROHS, as were disclosed in U.S. Pat. Nos. 6,797,758, 6,343,647, and 6,761,928, the alloy compositions thereof do not go beyond the research documentations on solder alloys. The documentations, for example, include the research of an Sn—In—Bi alloy (referring to H. Kabassis, J. W. Rutter, and W. C. Winegard: Metall. Trans., A 1984, Vol. 15A, ppl515-17. and Mater. Sci. Technol., 1986, Vol. 2, p. 985.) and the research of a Bi—In—Sn—Zn alloy (referring to Journal of Alloys and Compounds 360, 2003, pp 98-106, entitled “Thermodynamic optimization of the lead-free solder system Bi—In—Sn—Zn”). However, the lowest melting temperature of the above-referenced alloys (i.e. an eutectic temperature) is approximately 10° C. higher than the LMAs containing the hazardous elements such as Pb and Cd.
Eutectic In—Bi—Sn alloy TIM with components conforming to the RHOS and with the initial melting temperature at about 60° C. can merely develop heat dissipation performance on a condition that the junction temperature of the eutectic In—Bi—Sn alloy TIM exceeds 60° C., and thereby a liquid phase can then be generated to fill the air gaps or the voids in the interface. In other words, when the junction temperature is below the melting point of said alloy TIM, the air gaps and/or the voids are not filled, leading to a bottleneck in the heat conduction. In that situation, the eutectic In—Bi—Sn alloy TIM cannot be effectively applied to the newly developed microelectronic components having variable dynamic operating powers. Although metals of Ga and Ga—In—Sn liquid alloys have even much lower melting temperatures, they are not suitable for the metal TIM application, for they are in the liquid state under room temperature and thus are not easy to handle. Accordingly, it is desirable to develop and employ a non-hazardous metal TIM which has a lower melting temperature and achieves better heat dissipation performance within a wide junction temperature range, such that the bottleneck in the heat conduction can be avoided.